1. Most people can't tell the difference between 320kbps and lossless. If your files sound good they should be fine with any equipment. Although I have mostly CD quality FLAC, the 320kbps MP3s I have sound equally good.

If you want to take the test yourself, try this : https://www.npr.org/sections/therecord/2015/06/02/411473508/how-well-can-you-hear-audio-quality

2. Great cans will sound good with good files (which it sounds like you have). The transducer is the most important physical part of any setup (whether it is speakers or headphones) and should be the first thing invested in.

3. You should be hesitant to buy anything over, say, $500 from a purely audio quality standpoint. In my opinion, what you're buying past that point is build quality and comfort. That is just my opinion of course, so take it for what it's worth.

FWIW, I think the best deals for new cans below 500 bucks are the Dan Clark Aeon headphones on Drop (open and closed), and the Focal Elegia headphones on Adorama. You can find good deals on lightly used cans as well.